The present invention relates to packet switched network in-line tester and test method. In particular, the invention relates to a tester that is configured to generate packet delay variation by modifying a packet's timestamp according to a test scenario.
The performance of packet switched telecommunications networks has to be regularly tested. One way of doing this is to introduce known delay variations in a test scenario and monitor the network response. A standard approach to generating packet delay variation, also known as packet jitter, is based on modifying each packet's timestamp with a delay value according to a test profile. The test profile may be generated using a probability distribution, for example a Gaussian or uniform distribution. The test profile may be a function (e.g. step function, saw tooth) or may be otherwise defined by the user. Packets are stored in a buffer, alongside the modified timestamp, and sent out after the required delay time has lapsed.
In cases where the packet rate is relatively high, for example where the inter-packet gap (Δt) is smaller than the delay variation (ji, where i=0, 1, 2, . . . ) requested by the test profile, some packets are delayed long enough to affect subsequent packets. Those subsequent packets may then be delayed for longer than the delay time expected from the test profile. In these circumstances, the delayed packets are sent as soon as possible, but this can result in some packets being sent back to back. Case 1 below is an example where this problem is evident. This is illustrated in FIG. 1. Here, the inter-packet gap Δt is 6 and the packet delays range from 0 to 7.
Case 1Δt = 6Jitterprofile:Timestamps:j05→ t0′ = t0 + j0t5′ = t5 + j0j17→ t1′ = t1 + j1t6′ = t6 + j1j21→ t2′ = t2 + j2t7′ = t7 + j2j32→ t3′ = t3 + j3j40→ t4′ = t4 + j4
As can be seen from FIG. 1, for t1′ and t6′ some packets have a delay that affects following packets, and as a result packets are sent back-to-back. For example, for packet t2′ (t7′) the delay defined by the test profile should have been 1, but in practice because of t1′, the delay for t2′ is 2, which is longer than expected. For packet t4′ the minimum latency is achieved and the packet is not affected by jitter values from previous packets.
Case 2 below is another example where this problem is evident. This is illustrated in FIG. 2. As before, the packet delays range from 0 to 7. However, the packet rate is higher and the inter-packet gap is lower than the example of FIG. 1. In particular, the inter-packet gap is 3.
Case 2Δt = 3Jitterprofile:Timestamps:j05t0′ = t0 + j0t5′ = t5 + j0t10′ = t10 + j0t15′ = t15 + j0j17t1′ = t1 + j1t6′ = t6 + j1t11′ = t11 + j1j21t2′ = t2 + j2t7′ = t7 + j2t12′ = t12 + j2j32t3′ = t3 + j3t8′ = t8 + j3t13′ = t13 + j3j40t4′ = t4 + j4t9′ = t9 + j4t14′ = t14 + j4
In this example, t1′ is delayed by 7. This affects following packets, and so some packets are sent back-to-back. In particular, t1′, t2′, t3′ and t4′ are sent back to back. In this case, t4′ does not achieve minimum latency. Instead, packet t4′ is affected by jitter values from previous packets. As a result, all packets (t0′-t15′) are delayed, the minimum latency value is increased and the maximum jitter value is reduced. Some parts of the jitter profile become constant latency.
As demonstrated above, the standard approaches to jitter test do not guarantee that all packets are delayed for the time defined in the test profile. Instead packets are often delayed for longer. In addition, in some cases, the total packet delay variation may be reduced. This limits the accuracy of the test.